The present invention relates to a vertical semiconductor component and to a method for fabricating a drift zone of such a component.
Vertical semiconductor components of this type, which are described for example in U.S. Pat. No. 4,941,026 may be formed both as bipolar components, such as diodes, for example, or as unipolar components, such as MOS transistors or Schottky diodes, for example.
In the case of diodes, a second terminal zone of the second conduction type is arranged in the region of the second side of the semiconductor body opposite to the first side, the two complementarily doped terminal zones forming the anode and cathode zones or emitter and collector zones of the diode.
In the case of a vertical MOS transistor a second terminal zone—serving as a source zone—of the same conduction type as the first terminal zone—serving as a drain zone—is present, the source zone being separated from the drift zone by means of a body zone of the second conduction type. A gate electrode formed in a manner insulated from the semiconductor zones serves for forming a conductive channel in the body zone between the source zone and the drift zone.
What is crucial for the dielectric strength of such components, that is to say for the maximum voltage that can be applied between their load terminals before a voltage breakdown occurs, is the configuration, here in particular the doping and the dimensioning in the vertical direction, of the drift zone. The drift zone takes up the majority of the voltage present in the case of components of this type in the blocking state, that is to say in the case of a diode, when a voltage is applied which reverse-biases a pn junction in the anode and the drift zone, and, in the case of a MOS transistor, when a load path voltage is applied and the gate electrode is not driven. A reduction of the dopant concentration of the drift zone or a lengthening of the drift zone increases the dielectric strength, but is detrimental to the on resistance.
The provision of a field electrode arranged in a manner insulated from the drift zone and extending in the vertical direction of the semiconductor body, said field electrode being at a defined potential, compensates for charge carriers in the drift zone. This compensation effect affords the possibility of increasing the drift zone of the component compared with components without such a field electrode with the dielectric strength remaining the same, which in turn leads to a reduction of the on resistance.
U.S. Pat. No. 4,941,026 mentioned above describes putting the field electrode at a fixed potential, which, in the case of a MOSFET, may correspond to the potential at the gate electrode or to the source potential. The voltage loading of an insulation layer that insulates the field electrode from the drift zone varies in this case with the potential that changes in the vertical direction of the drift zone. Under the assumption that the field electrode is at the same potential as one of the load terminals—for example the source terminal in the case of a MOSFET or the anode terminal in the case of a diode-, the voltage loading of the insulation layer is particularly great in the vicinity of the other load terminal—that is to say the drain terminal in the case of a MOSFET or the cathode terminal in the case of a diode.
In order to take account of this potential distribution in the drift zone in the reverse-biasing case and in order to prevent a voltage breakdown from occurring between the drift zone and the field electrode at the locations of the insulation layer which are exposed to a high voltage loading, U.S. Pat. No. 6,365,462 B2 thus proposes varying the thickness of the insulation layer between the drift zone and the field electrode in the vertical direction such that it increases as the voltage loading increases. In the case of the known component, trenches running in the vertical direction of the semiconductor body are formed in the drift zone, zones made of polysilicon which taper in the vertical direction and serve as field electrodes being formed in said trenches. These zones are connected to the anode in the case where the component is configured as a diode, and form a lengthening of a gate electrode arranged in the trench in the case where the component is configured as a trench transistor.
A vertical component having a field electrode tapering in the vertical direction of the semiconductor body is also disclosed in U.S. Pat. No. 5,973,360.
U.S. 2003/0073287 A1 describes vertical power components having a drift zone, in which two field electrodes are provided in a manner spaced apart from one another in the vertical direction of the component, said electrodes being insulated from the drift zone. Said field electrodes are at different potentials during operation of the component.
For lateral components using SOI technology, DE 197 55 868 C1 discloses arranging a plurality of field plates that are insulated from the drift zone along the drift zone and connecting these field plates to sections doped complementarily with respect to the drift zone in the drift zone.